1. Field of the Invention
The present invention relates to electric signal filters used in electronic communications systems and, more particularly, pertains to intermediate frequency bandpass filter used to provide frequency selectivity in either a radio receiver or a radio transmitter.
2. Description of the Prior Art
Prior to the present invention, multiple bandwidth systems required a bank of fixed bandwidth filters, each composed of several quartz crystals and fixed passive components, and a switching scheme to select a filter with the desired passband. This was a costly physical circuit arrangement because of the duplication of circuitry in each of the unselected filters.
Crystal filters with circuit topologies similar to the present invention have been used before, but, given a set of quartz crystals, the design required a different impedance level for each design bandwidth, as described by M. Dishal, Modern Network Theory Design of Single-Sideband Crystal Ladder Filters, Proc. IEEE, Vol. 53, No. 9, Sept. 1965. Therefore, even though it was possible to use identical quartz crystals for different bandwidths, each filter required a different set of passive components of capacitors, inductors and transformers to achieve the desired bandwidth. This made each filter so unique that it was necessary to switch complete filter units to select different bandwidths.
An excellent discussion of crystal ladder filters is found in "Crystal Ladder Filters, How to Build Low-Cost s.s.b Filters Using Surplus Crystals" by J. Pocket, F6BQP, in Wireless World, July, 1977.
Typical crystal ladder filters - All crystals are of the same resonant frequency--preferably between 8 and 10 MHz for single side band units. Three and four-crystal filters are capable of giving very good results. Two-crystal filters, although reasonably good, have relatively poor shape factors. A filter of this kind can be made using two, three or four crystals in series. The choice of impedance is important because, in effect, the more this is reduced the more the passband is reduced and the higher will be the insertion loss. This is because the series resistance of the crystal becomes more significant in relation to the impedance.
On the other hand, if one chooses an impedance which is too high, the calculations will result in low capacitance values, and construction then becomes limited by the stray circuit capacitances.
In practice, for a frequency of about 8 to 10 MHz, the impedance should be about 800 to 1000 ohms to obtain a passband of 2100 Hz, suitable for s.s.b.
It is necessary to underline the importance of the impedance of a filter no matter what type is used. It is also of paramount importance that the filter should be correctly terminated because any significant mismatch could lead to a passband ripple of some 10 dB.
It is possible to adjust the values of the capacitors; reducing them increases the passband but also increases the ripple in the passband. If a ripple of 2 dB can be accepted, the passband can be increased by up to 20%. Note that it is advisable not to take advantage of this opportunity unless the necessary test instruments are available to check the results of any such adjustments (a wobbulator and oscilloscope are the ideal instruments for this type of adjustment). By using a lower frequency and lower impedance, it is possible to make an excellent CW filter.
The present invention overcomes the disadvantages of the prior art by providing a crystal filter with varactor diodes where the width of the filter passband is set by a DC voltage through a resistor biasing network on a single control line.